Photographic processes for obtaining direct positive images without employing a reversal processing step or a negative film are well known.
Among direct positive photographic processes, a process for obtaining a direct positive image comprising exposing imagewise an internal latent image type silver halide emulsion not having been previously fogged and the conducting surface development either after a fogging treatment or along with a fogging treatment is preferred from the standpoint of obtaining a relatively high sensitivity.
The term "internal latent image type silver halide photographic emulsion" as used herein means a silver halide photographic emulsion of the type in which light-sensitive nuclei are mainly present in the interior of silver halide grains and a latent image is formed mainly in the interior of the grains by exposure.
With respect to these processes, various techniques have been hitherto known and main techniques include those described, for example, in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, British Patents Nos. 1,151,363, 1,150,553, 1,011,062, etc.
In order to form direct positive images, either after an internal latent image type silver halide photographic material is subjected to a fogging treatment using a light fogging method or a chemical fogging method or while an internal latent image type silver halide photographic material is being subjected to the fogging treatment, surface color development is conducted, followed by bleaching and fixing (or bleach-fixing), if desired. After fixing, generally, washing with water and/or stabilizing are performed.
In the direct positive image forming method using the fogging treatment, the developing speed is low and a long period of processing time is required as compared with the case of processing conventional negative type photographic materials. Therefore, in order to shorten the processing time, it has been heretofore performed employing a developing solution having a high pH and/or a high temperature. However, it is also generally known that the minimum image density of direct positive images obtained increased when developed at a high pH.
Other methods for increasing the developing speed in direct positive image formation are also known. For example, there is a method to employ a hydroquinone derivative described in U.S. Pat. No. 3,227,552 or a method to employ a mercapto compound having a carboxylic acid group or a sulfonic acid group described in JP-A-60-170843 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, these compounds exhibit only a small effect on the increase in developing speed, and the technique for effectively increasing the maximum density of direct positive image has not been found. Therefore, it has been desired to develop a technique which provides a sufficiently high maximum image density in a short period of time even when processed with a developing solution having a low pH.
In the case wherein nucleating development is performed in the presence of a nucleating agent using a chemical fogging method, it is know to ordinarily employ a quaternary heterocyclic compound or a hydrazine type compound as a nucleating compound.
However, when the quaternary heterocyclic compound is employed alone as a nucleating agent, a minimum image density is apt to increase along with the increase in maximum image density, and its stability under high temperature and high humidity conditions or a high temperature and low humidity conditions is poor, resulting in a decrease in the maximum image density. Further, it has a problem in that its efficiency changes widely depending on the variation of the pH of the developing solution.
On the other hand, when the hydrazine type compound is employed alone as a nucleating agent, the minimum image density increases as described above because of using a high pH condition required to obtain a sufficient maximum density. It has also a further problem in that the developing activity of the developing solution severely decreases.
Although a technique wherein two or more kinds of hydrazine type compounds are employed together is known as described in British Patent No. 2,107,074, the above described problem can not be solved at all by the use of two or more hydrazine type compounds having almost the same nucleating rate.